Since the discovery of electrical conductivity of conjugated polymers upon doping, conjugated polymers have grown to be one of the most active research fields in materials science in the past two decades. Conjugated polymers are a novel class of semiconducting materials that combine the optical and electronic properties of semiconductors in general with the processing advantages and mechanical properties of polymers. As the active media, conjugated polymers have been used in the fabrication of many photonic devices, including diodes, light-emitting diodes, photodiodes, polymer grid triodes, field-effect transistors, light-emitting electrochemical cells, and lasers. Significant progress has been made in the development of polymer light emitting-diodes as an emerging display technology. Emission over the whole visible spectrum has been demonstrated with impressive efficiency and brightness. By using a metallic polymer as a hole-injecting contact, robust and mechanically flexible LEDs have been demonstrated. For fall-color displays, efficient red, green, and blue electroluminescnet polymers are required.
However, although all the three colors have been demonstrated in polymer LEDs, only red (orange) such as from poly(2-methoxy-5-(2′-ethyl-hexyloxy)-para-phenylenevinylene) (MEH-PPV) (see U.S. Pat. No. 5,189,136) and green such as from poly(1,4-phenylene vinylene) (PPV) (see U.S. Pat. No. 5,399,502) have sufficient efficiency and stability to be of commercial value.
The first blue polymer LED was fabricated from poly(para-phenylene) (PPP). Subsequently, soluble PPP derivatives were synthesized and used to produce blue polymer LEDs. The PPP derivatives exhibit high photoluminescence (PL) and electroluminescence (EL) efficiencies, but low molecular weights and poor stability of the polymers limit their application in polymer LEDs. Other blue EL polymers include derivatives of PPV, 3,4-disubstituted poly(thiophenes), and some copolymers with conjugated or non-conjugated segments. Although some polymer blends have also been studied for blue electroluminescence, almost none of the polymers has demonstrated good EL performance. In the past few years, 9,9-disubstituted polyfluorenes have attracted considerable attention as blue EL materials. The synthesis of soluble polyflurenes was first disclosed by Fukuka et al. [J. Polym. Sci., Polym. Chem. Ed., Vol. 31, p. 2456 (1993)]. The polymers were obtained by oxidative polymerization, which is not regiospecific and normally results in low degrees of polymerization. Thereafter, the synthesis of well-defined poly(2,7-fluorene) derivatives from 2,7-dihalofluorenes by a nickel (0)-catalyzed coupling reaction was disclosed independently by Woo et al. (WO 97/05184) and Pei et al. (WO 97/33323). More recently, high molecular weight poly(9,9-dioctyl-2,7-fluorene) was synthesized by Inbasekaran et al. through a modified Suzuki coupling reaction (U.S. Pat. No., 5,777,070). However, sufficient stability of the polymer shown in U.S. Pat. No. 5,777,070 has not been demonstrated. Moreover, the emissive spectra of these polymers normally correspond to a whitish blue instead of pure blue, and this becomes more serious with the operation of EL devices. Thus, there is a need for a more efficient and stable pure blue EL polymer for use, for example, in the fabrication of blue polymer LEDs to realize full-color displays, and it is an object of the present invention to provide this.
Although a degree of blue light emission has been demonstrated by some conjugated polymers, their application in polymer LEDs, considering the efficiency, stability, and processability required, has been mainly focused on substituted poly(para-phenylenes) (PPPs) and 9,9-disubstituted polyfluorenes (PFs). The most attractive properties of PPPs are their high PL and EL efficiencies, but it is difficult to get high molecular weight in PPPs and the stability of PPPs is poor. These serious drawbacks have limited the applications of PPPs in polymer LEDs and other optoelectronic devices. Over the past few years, the efforts of developing efficient and stable blue EL polymers for polymer LEDs have been focused on PFs. High molecular weight and high purity PFs have been successfully synthesized by the Dow Chemical Company and high EL efficiency, high luminance, and low driving voltages have been demonstrated in the LEDs fabricated from PFs. Unfortunately, the emission colors of PFs are normally not pure blue, but whitish or even greenish blue because of the formation of interchain excimers, and the problem of color impurity becomes more serious with the operation of the devices. Having this drawback, PFs are not suitable for the realization of full-color displays.
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Insofar as prior patent documents are concerned, an important patent related to the synthetic process of the polymers of the present invention (but which is not related to the polymers themselves) is U.S. Pat. No. 5,777,070.
Other patents related to the general field of the present invention are U.S. Pat. No. 5,708,130 and WO 97/05184 (“2,7-Aryl-9-substituted fluorenes and 9-substituted fluorene oligomers and polymers”), U.S. Pat. No. 5,900,327 and WO 97/33323 (“Polyfluorenes as materials for photoluminescence and electroluminescence”), U.S. Pat. No. 5,807,974 (“Fluorene based alternating copolymers for electroluminescence element and using such copolymers as light emitting materials”), and U.S. Pat. No. 5,876,864 (“Fluorene based alternating copolymers containing acetylene group and electroluminescence element”).